1. Field of the Invention
This invention relates to motion activated lights.
2. Description of the Related Art
Passive infrared (PIR) sensors have been developed and utilized in various devices to sense the approach or motion of a person. Infrared radiation is the physical means of detection, there being a known relationship between the wavelength of the strongest infrared radiation and the temperature of the body emitting it. Thus, the body of a human, as well as that of some other animals, radiates the strongest infrared radiation between 9 .mu.m and 10 .mu.m. Infrared radiation can be detected due to the "pyroelectric effect", which is due to the generation of a surface electric charge on certain dielectric crystalline materials when exposed to infrared radiation, which differs from the "natural charge" at thermal equilibrium that is present due to spontaneous polarization. The current which flows when there is a temperature change and resultant change in surface charge can be measured by means known in the art, for example, by connecting a high impedance resistor between the electrodes of both crystal surfaces and reading the voltage drop.
An improvement in infrared detection devices occurred when the optical system was developed to include a number of facets in an associated reflector and/or one or more lenses, which were oriented so that radiation originating in the particular ranges of angular scan of the lenses was sequentially directed to a group of thermal detectors (See for example, U.S. Pat. No. 3,958,118 of Schwartz).
Motion activated light fixtures now generally utilize infrared sensors, together with an optical collecting and focusing means such as a system of Fresnel lenses. The aggregate of the individual fields of view of the multiple Fresnel lenses defines the overall field of view of the device. Generally, the lens plates to which the Fresnel lenses are mounted are curved sections from a cylindrical surface, with the Fresnel lenses being mounted in parallel rows. Examples include various fixtures sold by Regent Lighting Corporation (e.g., Model Numbers MS35, MS30 and MS80). Fresnel lenses, each of which is made of a surface of stepped concentric circles, collect radiation from a moving object and direct the energy to a detector. The detector can be a thermistor in which resistance changes with a change in the energy level, or more generally, a pyrosensor which generates a voltage or alters a current passing through it. Such fixtures also comprise one or more signal amplifiers and a control circuit.
Current motion activated light fixtures, particularly those with two bulbs, typically have a junction box coverplate to which the two light bulbs are mounted as shown in FIG. 1. Mounted between the two light bulbs is a protruding external sensor S which is adjustable in position so that it can be aimed in various directions to detect motion when the fixture is mounted on the eaves or ceiling or on a wall. Because this external sensor protrudes from the fixture, it is subject to damage due to impact of flying or passing objects. Also, it often detracts from the appearance of the fixture. The very visible sensor also gives notice to an intruder that there is a motion detector attached to the light fixture.
Light fixtures with integral motion detectors which do not protrude have been designed so that the lens is placed to avoid viewing interference from other parts of the fixture. See for example, the wall light fixture of Lee (U.S. Pat. No. 5,282,118). Because of the fixed position of the detector of such fixtures, the angular range of detection of this fixture is limited.
It is therefore an object of this invention to provide a motion activated light fixture which has a fixed sensor head which does not protrude from the fixture but is integrated into the junction box coverplate.
It is a further object of the invention to provide a motion activated light fixture which allows detection range flexibility whether mounted to vertical or horizontal surfaces without having a movable external sensor head.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.